Frequently, broadband systems transmit television signals to subscribers of a conditional access system. Broadband systems, such as cable and satellite television systems, typically include a headend for receiving programming, or sessions, and/or services from various sources and redistributing the programming and/or services through a distribution system to subscribers. The headend receives programming signals from a variety of sources, combines the programming signals from the various sources, and transmits the combined signals through the distribution system to subscriber equipment. The distribution system can include a variety of media, such as coaxial cable, fiber optic cable, and satellite links, as well as a network of distributed nodes that then transmit the programming to subscriber locations, or to a network of distributed hubs, which transmit the signals to subscriber equipment, or any combination thereof.
In a cable television system, the subscriber equipment can include, among others a cable-ready television, a cable-ready video cassette recorder (VCR), or a digital home communications terminal (DHCT) that is connected to a television, computer, or other display device. The headend typically uses modulators to provide the streams of data in RF signals to the distribution system. For certain implementations, the Data Over Cable Service Interface Specifications (DOCSIS) apply. DOCSIS as referenced in this application is described in the DOCSIS 1.0 standards (SP-RFI-105-991105) and DOCSIS 1.1 standards (SP-RFlv1.1-106-001215), which are incorporated herein by reference. DOCSIC specifies the protocol for exchanging bidirectional signals over cable and specifies a cable modem termination system (CMTS) which provides the cable plant RF physical layer functionality required for CATV headends.
The DOCSIS forward data channel is implemented such that DOCSIS signals can use existing modulators intended for Moving Picture Experts Group (MPEG) transport. This is accomplished by carrying the DOCSIS data frames within the 184 byte payload of a 188 byte MPEG transport packet. The four byte header is defined to be compatible with the MPEG packet header. The MPEG packet header contains a 13 bit number used for identification, known as the packet identification or PID. A well known PID is defined for use in carrying all DOCSIS data. MPEG transport streams include overhead information such as MPEG tables that indicate the types and location of the programming within the transport stream. MPEG as referenced in this application is described in the MPEG-1 and MPEG-2 standards. The MPEG-1 standards (ISO/IEC 11172) and the MPEG-2 standards (ISO/IEC 13818) are described in detail in the International Organization for Standardization document ISO/IEC JTC1/SC29/WG11 N (June 1996 for MPEG-1 and July 1996 for MPEG-2), which is hereby incorporated by reference.
A typical CMTS uses dedicated forward data channel quadrature amplitude modulation (QAM) modulators that are connected directly to the Media Access Control (MAC) functions such that the MAC functions output directly into the QAM modulators. In order to synchronize reverse channel transmissions from subscriber devices, a time reference is transmitted to each subscriber device (including a subscriber modem) cable through the forward QAM data channel. This time reference or “SYNC message” is generated in the media access control function and is typically the value of a 32 bit counter clocked by a master 10.24 MHz clock, for example, in the CMTS. Each subscriber device receives this message and synchronizes an internal clock and counter to this reference. In order for this time reference to work, it must be transmitted with a constant or near constant delay through the network.
It is desired to provide an interface on the CMTS that would allow a generic and/or non-integrated QAM modulator to be used for the forward channel data in a DOCSIS application. It is also desirable to meet the constant transport delay constraint even if multiple information streams are combined.